Zeolites are microporous crystalline materials formed by a TO4 tetrahedral network wherein all tetrahedrons share their vortexes creating a three-dimensional structure that contains molecule-sized channels and/or cavities. They have a variable composition and T generally represents atoms in a +3 o +4 formal oxidation state such as Si, Ge, Ti, Al, B, Ga, . . . When any of the T atoms is in an oxidation state lower than +4, the crystalline lattice that is formed displays negative charges that are compensated by the presence of organic or inorganic cations in the aforementioned channels or cavities present in the structure. Said channels and chambers may also house organic molecules and H2O, and therefore, and in a general manner, the chemical composition of zeolites may be represented by the following empirical formula:x (M1/nXO2):y YO2:z R:w H2Owhere M is one or several organic or inorganic cations with +n charge; X is one or several trivalent elements; Y is one or several tetravalent elements, usually Si; and R is one or several organic substances. Although by means of post-synthesis treatments the nature of M, X, Y and R and the values of x, y, z, and w can be changed, the chemical composition of a zeolite (as synthesized or after calcinations) has the characteristic range of each zeolite and of the method used to obtain it.
The crystalline structure of each zeolite has a species-specific system of channels and cavities. This channel and cavity system originates a characteristic pattern of x-ray diffraction that serves to differentiate them.
Many zeolites have been synthesized in the presence of an organic molecule that acts as a structure defining agent. These organic molecules that act as structure defining agents (SDAs) contain, generally, nitrogen and can originate stable organic cations in the reaction medium.
The mobilisation of the precursor species during zeolite synthesis can be done in the presence of hydroxyl groups and basic media, that can be introduced as a hydroxide of the SDA used, such as for example tetrapropyl ammonium hydroxide in the case of ZSM-5 zeolite. Also, the fluoride ions can act as mobilising agents during zeolite synthesis. Patent EP-A-337479, for instance, describes the use of HF in H2O at low pH as mobilising agent of silica in ZSM-5 zeolite synthesis.